The present disclosure relates to a radio frequency switching circuit with advanced isolation characteristics, for application to a communications system.
In general, a semiconductor integrated circuit installed in a communications system includes a radio frequency switching circuit for controlling a transfer path of a radio frequency signal between an antenna and transmitter/receiver. Radio frequency switching circuits are commonly used in communications systems such as Bluetooth™, cellular personal communications services (PCS)/code division multiple access (CDMA)/wideband code division multiple access (WCDMA)/time division multiple access (TDMA)/global system (GSM)/standard for mobile communications, and other communications schemes, as well as in wireless local area networks (LAN).
In general, a radio frequency switching circuit is used between a transmitter and a receiver in various communications systems using time-division multiplexing (TDM). By virtue of this radio frequency switching circuit, the transmitter and the receiver are alternately turned on and off, thereby reducing overall power consumption of a system and also reducing interference between the transmitter and the receiver.
For the radio frequency switching circuits, low insertion loss, high degrees of isolation, high switching speeds, and high power handling capabilities are required, and low harmonic distortion, that is, a high degree of linearity, is required even if a large signal is input.
Among these characteristics, as insertion loss characteristics are enhanced, degradations in the sensitivity of a receiver and a transmitter power loss during transmissions, due to a switch may be reduced. Power handling capability ensures the maximum output of power from the transmitter. In addition, high isolation characteristics minimize influence upon the receiver that is turned off when the transmitter operates, and vice versa.
In particular, in a mobile communications system, among the aforementioned characteristics, insertion loss and power handling capabilities are more required.
In a conventional wireless communications apparatus, a radio frequency switching circuit includes a plurality of radio frequency ports connected to a plurality of transmitters/receives, respectively, and a common port connected to antennas.
The radio frequency switching circuit controls a transfer path of a radio frequency signal between the plurality of radio frequency ports and the common port to select one of the plurality transmitters/receivers connected to the radio frequency switching circuit to be electrically connected to an antenna.
A conventional radio frequency switching circuit may include a switching circuit portion connected between each radio frequency port and a common port and a shunt circuit portion connected between each radio frequency port and a ground in order to convert a transfer path of a radio frequency signal between each radio frequency port and the common port.
In this case, the switching circuit portion may include a transmission switching circuit portion Tx SW and a reception switching circuit portion Rx SW, and each of the transmission switching circuit portion and the reception switching circuit portion may include a plurality of semiconductor switches.
The semiconductor switch may be a switching device formed on a silicon on insulator (SOI) substrate and may include a metal oxide silicon field effect transistor (MOSFET).
In a conventional radio frequency switching circuit, in order to prepare a case in which a signal having a voltage higher than a breakdown voltage of one transistor is applied, the switching circuit portion includes a structure obtained by stacking a plurality of transistors.
In the structure obtained by stacking a plurality of transistors, a voltage higher than a rated voltage is divided and applied to a plurality of transistors, and thus, a voltage applied to one transistor is reduced to protect transistors from a high voltage.
In the conventional radio frequency switching circuit, agate signal Vg having a voltage higher or lower than a threshold voltage Vth of each transistor included in the transmission switching circuit portion and the reception switching circuit portion is applied to a gate of each transistor so as to control the transistor to be in an on-state or an off-state. The gate signal Vg may be provided by a base band chip set.
However, in order to ensure isolation required between the transmitter and the receiver, the conventional radio frequency switching circuit includes a shunt circuit portion for bypassing unnecessary signals to a ground in addition to the switching circuit portion for selection of a signal path.
In addition, the conventional shut circuit portion includes a plurality of transistors stacked in series, and thus, problems arise in that a product size is increased.
Patent Document 1 of Related Art Document below discloses a radio frequency switch and a semiconductor apparatus but does not disclose technical features for improving isolation while replacing a shunt circuit portion acting as a burden due to a large size thereof.